This invention relates generally to the maintenance of nuclear steam supply systems. More particularly, this invention relates to a technique of ultrasonically examining remotely located welds in cast stainless steel nuclear steam supply systems.
FIG. 1 illustrates a nuclear steam supply system 20 in accordance with the prior art. As known in the art, the system 20 includes a reactor vessel 22 with a core 24. The system 20 also includes a set of coolant loops 26A-26D. Each coolant loop 26 includes a steam generator 28 and an associated reactor coolant pump 30. Each steam generator 28 includes a feedwater inlet 32 and a steam outlet 34.
The operation of a nuclear steam supply system 20 is well known in the art. Coolant leaving the reactor vessel 22 enters a steam generator 28 where it imparts its heat to a working fluid which exits as steam through a steam outlet 34. The steam is then used to drive a turbine (not shown) to produce electricity. The coolant leaves the steam generator 28 via the reactor coolant pump 30 and is pumped back to the reactor vessel 22.
The present invention is directed toward the maintenance of reactor coolant pumps 30 associated with nuclear steam supply systems. FIG. 2 is a cut-away view of a prior art reactor coolant pump 30. The reactor coolant pump 30 includes a pump shaft 40 connected to an impeller 42. A seal housing 44 surrounds the pump shaft 40. The seal housing includes a set of seal injection lines or pipes 46A, 46B.
The reactor coolant pump 30 also includes a casing 48. A set of cooling lines or pipes 50A, 50B pass through the casing 48 and into a thermal barrier 52. The casing 48 and thermal barrier 52 are formed of cast stainless steel.
FIG. 3 is a top view of a cooling line 50 passing into the casing 48. FIG. 4 illustrates the seal injection line 46 terminating in a thermal barrier 52. A weld 60 is used to connect the seal injection line 46 to the thermal barrier 52.
The present invention is directed toward identifying a flaw 62 associated with a weld 60. The flaw may be in many forms, for example, a crack or an incomplete weld penetration. The flaw may be in a cooling line 50, a seal injection line 46, or any other remote location. The invention is most useful in relation to cast stainless steel components associated with nuclear steam supply systems.
There are no known prior art techniques for identifying flaws in attachments to reactor coolant pumps of nuclear steam supply system coolant loops. More particularly, there are no known techniques for identifying flaws in seal injection lines and cooling lines of reactor coolant pumps. The present practice is to wait for a failure and then shutdown the plant. Repairs and welds are then made during shutdown. The expense associated with an unplanned shutdown of this type is typically about $500,000 per day. Thus, it would be highly desirable to provide a technique for detecting flaws in remotely located nuclear steam supply system components. Such a technique would save critical operation time and prevent unscheduled plant outages.
It is difficult to identify defects of the type described above for a number of reasons. First, the defects are remotely located. Therefore, a special apparatus must be contrived to reach the remote location. Another problem is that the seal injection lines 46 and cooling lines 50 are connected to cast stainless steel components.
Conventional ultrasonic examination of such components is not possible for the following reasons. First, the properties of the cast stainless steel material from which the thermal barrier is fabricated are not conducive to the transmission of ultrasonic energy due to large gain structure, ultrasonic beam redirection, and ultrasonic scattering. Second, access to the thermal barrier for ultrasonic examination is limited by component geometry. Third, the welds are typically not examined due to inherent reflectors formed during fabrication of the weld joint. Fourth, the orientation of the crack is not conducive to a conventional examination procedure, even if the material properties would permit ultrasonic transmission and subsequent reflection from the crack face.
Cast stainless steel material typically consists of large, randomly orientated grain structure that tends to scatter and otherwise disperse ultrasonic energy. The nuclear power industry has spent a great deal of money trying to identify ultrasonic examination techniques for cast stainless steel material, with very limited success. Present techniques for ultrasonic examination of cast stainless steel material is unreliable, at best. The most promising results, although very limited, have been obtained by using low frequency, large diameter, dual-element ultrasonic transducers. The available access to the thermal barrier is inadequate for this size of probe. In addition, even if the ultrasonic energy were to reach the crack location, the ultrasonic wave length would be too large (with low resolution) for efficient reflection from the small crack face. The beam size of conventional probes is also larger than this particular weld and would result in an ineffective examination.
In view of the foregoing, it would be highly desirable to provide an ultrasonic examination technique to detect remotely located flaws in nuclear steam supply systems prior to leakage. Such a technique would allow repair of the flaws during scheduled refueling outage, instead of during unplanned outages.
An apparatus for ultrasonically examining a weld in a nuclear steam supply system includes an elongated guide rod for positioning within a pipe of the nuclear steam supply system. The pipe may be, for example, a seal injection line or cooling line of a reactor coolant pump. An ultrasonic transducer is positioned at an end of the elongated guide rod. A collapsible shoe encloses the ultrasonic transducer. The collapsible shoe includes a biasing mechanism to allow the collapsible shoe to pass through the pipe while the pipe is at a first circumference and while the pipe is at a second circumference. The collapsible shoe continuously contacts the pipe to establish ultrasonic coupling for the ultrasonic transducer.
The invention can detect weld flaws without the need of passing the ultrasonic beam through the complex grain structure of the cast stainless steel. The invention can be used to identify weld flaws during scheduled outages so that unexpected damaging weld leaks and concomitant plant outages can be avoided.